1. Field
The present application relates to compositions and methods for preventing dental caries. In particular, the present application relates to aqueous compositions and methods for improving the mechanical strength of teeth.
2. Description of Related Art
Dental caries are one of the most common preventable diseases plaguing humans and non-human animals worldwide. Dental caries (tooth decay, cavity) is caused by bacterial processes which damage hard tooth structure (e.g., enamel, dentin, and cementum). These tooth structures break down progressively, producing holes in teeth (dental caries). Two groups of bacteria—Streptococcus mutans and Lactobacillus spp.—produce lactic acid in the presence of fermentable carbohydrates such as sucrose, fructose, and glucose, and are largely responsible for initiating caries. Teeth, which are comprised primarily of minerals, are constantly subjected to demineralization and remineralization between teeth and the surrounding saliva. Hydroxylapatite—a crystalline calcium phosphate—is the primary mineral of a tooth's enamel surface and has the general formula Ca5(PO4)3(OH), but is usually written Ca10(PO4)6(OH)2, to denote that the crystal unit cell comprises two entities. When the pH at the tooth surface drops below 5.5, demineralization proceeds faster than remineralization (i.e. a net loss of hydroxylapatite on the tooth's surface occurs). This loss of mineral structure results in tooth decay. If left untreated, the disease can lead to pain, tooth loss, infection, and, in extreme cases, death.
To prevent dental caries, dental professionals recommend brushing teeth at least twice a day with a fluoride-containing dentifrice, which removes bacterial plaques. Fluoride assists in the prevention of caries by binding to hydroxylapatite surfaces in the tooth enamel, forming fluorapatite and making the enamel more resistant to demineralization (and thus more resistant to decay). Fluoride is also commonly added to municipal water supplies to prevent tooth decay, yet the National Research Council (functioning under the auspices of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine) recently concluded that the levels of fluoride found in municipal water supplies and the levels of fluoride known to cause toxic effects are dangerously close to one another.
There has been little to no innovation in commercial toothpastes since the mid-twentieth century. The active anti-caries ingredient in all commercial toothpastes is either 0.24% sodium fluoride (NaF, 0.15% w/v fluoride ion), or 0.76% sodium monofluorophosphate (Na2FPO3, 0.14% w/v fluoride ion). Recent innovations in the commercial toothpaste market are directed to flavors, abrasives, or whiteners, instead of cavity-fighting activity.
Theobromine (IUPAC name: 3,7-dimethyl-2,3,6,7-tetrahydro-1H-purine-2,6-dione; also known as 3,7-dimethylxanthine) is a white (or colorless) bitter-tasting crystalline powder with a sublimation point of 290-295° C., a melting point of 357° C., and a molecular weight of 180.16 g/mol. The solubility of theobromine in water is 1.0 g/2 L; in boiling water, it is 1.0 g/150 mL, and in 95% ethanol it is 1.0 g/2.2 L. Theobromine is related chemically to caffeine and theophylline, and is found in numerous foods including chocolate, cocoa, tea leaves, and acai berries. The chemical structures of theobromine, theophylline (1,3-dimethylxanthine), and caffeine (1,3,7-trimethylxanthine) are given below as formulae I, II, and III, respectively.
Theobromine is found naturally in cacao beans (Theobroma cacao) at a concentration of from about 1.5% to about 3%, and in the husk of the bean at a concentration of from about 0.7% to about 1.2%, or about 15 to about 30 g/Kg (Winholdz, 1983). Though part of the same chemical family, one must distinguish the stimulant effects of theobromine from those of caffeine. Caffeine acts relatively quickly, and its main effect on humans is increased mental alertness; theobromine's effect is subtler, and causes a mood elevation that is milder longer-lasting than that of caffeine. Theobromine's plasma half-life (t1/2) in the bloodstream is six hours, while caffeine's is only two hours. Another difference is that theobromine is not physiologically addictive, producing no withdrawal symptoms after prolonged regular consumption, while caffeine has been proven to be physiologically addictive and linked with many cases of proven withdrawal.
Two independent studies conducted in the 1980's found that the average level of theobromine in eight varieties of commercial cocoa powder was 1.89% (Shively & Tarka, 1984 and Zoumas et al., 1980). Of particular relevance are the normal levels of theobromine found in commercially-available foodstuffs, shown below in TABLE 1.
TABLE 1Food TypeTheobromine Contenthot chocolate beverages65mg/5-oz servingchocolate milk (from instant or sweetened58mg/servingcocoa powder)hot cocoa (average of 9 commercial mixes)62mg/servingcocoa cereals*0.695mg/gchocolate bakery products*1.47mg/gchocolate toppings*1.95mg/gcocoa beverages*2.66mg/gchocolate ice cream*0.621mg/gchocolate milk*0.226mg/gchocolate pudding*74.8mg/servingcarob products*0-0.504mg/gSources: Zoumas, et al., 1980; Blauch & Tarka, 1983; Shivley & Tarka, 1984; Craig & Nguyen, 1984.*Theobromine content determined by HPLC/reverse-phase column chromatography
Dark chocolate contains the highest levels of theobromine per serving of any type of chocolate, but the concentrations tends to vary between about 0.36% and about 0.63%. To put this into perspective with the foodstuffs mentioned in TABLE 1, a one-ounce bar of dark chocolate contains about 130 mg of theobromine, while a one ounce bar of milk chocolate contains about 44 mg of theobromine. Thus, the concentration of theobromine in a one-ounce bar of dark chocolate is approximately two times the amount in a 5-ounce cup of hot chocolate. For a 143-pound human being to achieve a toxic level of theobromine in their blood, they would have to ingest approximately 86 one-ounce milk chocolate bars in one sitting.
Theobromine can also be isolated or produced as an amine salt (e.g., the ethylene diamine salt thereof) or a double salt thereof (e.g., with alkali metal salts or alkaline earth metal salts of organic acids, for example alkali or alkaline earth metal salts of acetic, gluconic, benzoic, or salicylic acid). The double salts may be prepared either to make the theobromine more water souble, or to make insoluble complexes.
In 1966, Strålfors reported a reduction of dental caries in hamsters that were fed diets rich in chocolate. The Strålfors study examined the effect on hamster caries by comparing cocoa powder, defatted cocoa powder, and cocoa fat. Pure cocoa powder inhibited dental caries by 84%, 75%, 60%, and 42% when the cocoa powder comprised 20%, 10%, 5%, and 2% percent of the hamster diet, respectively. Defatted cocoa showed a significantly higher anti-caries effect than did fat-containing cocoa powder, but cocoa butter alone (comprising 15% of the hamster's diet) increased dental caries significantly (Strålfors A. “Effect on Hamster Caries by Dialyzed, Detanned or Carbon-treated Water-Extract of Cocoa” Arch Oral Biol. 1966; 11:609-15.
In a follow up study, Strålfors studied the nonfat portions of the cocoa powder and demonstrated that cocoa powder washed with water possessed considerably less anti-cariogenic effect than unwashed cocoa powder. Nevertheless, Strålfors still observed a considerable anti-caries effect in the washed cocoa powder group, “indicating an existence of a non-water soluble cariostatic factor,” and alluded to the existence of “two caries-inhibitory substances in cocoa: one water-soluble, and another which is sparingly soluble in water” (Strålfors, A., 1966).
Subsequent studies suggested that apatite crystals grown in vitro in the presence of theobromine were significantly larger than those grown in the absence of theobromine (see, e.g., U.S. Pat. Nos. 5,919,426 and 6,183,711, each of which is incorporated by reference in its entirety). Ingestion of theobromine by lactating rats was correlated with increased hydroxylapatite crystallite size (higher crystallinity) in the whole first molars of nursing pups exposed to theobromine, versus controls, as well as increased resistance to acid dissolution (see id.). The femurs of nursing female pups exposed to theobromine demonstrated higher crystallinity, and were stronger and stiffer than gender-matched controls; the femurs of male pups, however, did not show this relationship (see id.).
The Hall-Petch relationship, however, dictates that the resistance of a solid material to permanent deformation (e.g., its indentation hardness) increases as the particle size decreases. Consequently, the increased hydroxylapatite crystallinity observed after exposure to theobromine—coupled with the Hall-Petch relationship—suggests that the resistance of bone and teeth to indentation and permanent deformation should decrease after exposure to theobromine due to the larger crystal size. This suggestion finds support in recent work, demonstrating that “the hardness of [hydroxylapatite] follows the Hall-Petch relationship as the grain size decreases from sub-micrometers to nanometers” (Wang J. et al. “Nanocrystalline hydroxyapatite with simultaneous enhancements in hardness and toughness” Biomaterials. 2009; 30:6565-72). Another study suggests that the “hardness” of hydroxylapatite has little to do with particle size, showing almost no change in hardness with decreasing grain size, yet demonstrates that the “fracture toughness” of hydroxylapatite is increased with decreasing particle size (Mazaheri M, et al. “Effect of a novel sintering process on mechanical properties of hydroxyapatite ceramics.”J. Alloys Compd. 2009; 471:180-4). A study of human adult and primary (baby) teeth demonstrated that, “[w]hen compared to the adult tooth, the baby enamel was thinner, softer, more prone to fracture, and possessed larger [hydroxylapatite] grains” (Low I M, et al “Mapping the structure, composition and mechanical properties of human teeth.” Mater Sci Eng C Mater Biol App. 2008; 28:243-47.).
Because of such conflicting and paradoxical results, one cannot extrapolate the known characteristics and responses of hydroxylapatite to environmental factors to predict a reliable or accurate result cannot be predicted simply by evaluating the prior art and extrapolating a result.